In the preview monitor, I keep an eye on FPS-variations. And I don't understand the reason why it varies so much - Same settings (I think) can give me from 20 to 55FPS. It comes and goes. Doing an orderly start up seems to help a little, but there must be more to it.
Normally I input 2 cameras, HD SDI 720/50P, via BM ingesters. And mess around with 5-6 layers on top of a live feed from the cameras.
I'm on a 2016 Macbook pro with a Radeon Pro 460 built in + a Sonnet Puck with a 570 in it. The puck works fine (I use the "set egpu" SW in Terminal)
- It seems that the FPS-variations I struggle with are only very little affected by using the eGPU or not.

Any inputs on this is highly appreciated (and don't tell me to just throw away the bloody Mac, I know;)

FPS dips can be caused by various things, but they are always a bottleneck in the system.
If it happens with the combination of specific effects, and is a steady FPS value, then the effects are probably too heavy for your GPU to handle.
Or the machine could be thermal throttling.

If you see momentary dips, then it could be some background task, or another program eating up system resources for a short time. Disk read bursts, or CPU usage spikes at the same time as the dips, could be a sign of this.

If it's a thermal issue then the composition setting's framerate setting may help you in stabilizing the fps. You'll have to figure out how much it can run without ever throttling but you could try setting it to 30 and then see if it still drops down to 20. If it does then you can set it to 20 and that should make it stable again.